.substring error: "is not a function"

I don't understand why I get an error message using the substring method to declare a variable.

I want to use the first part of the URL in a comparison.

Site: http://www.elizabet.nl/wordpress

This is the part that's going wrong:

var currentLocation = document.location,
muzLoc = currentLocation.substring(0,45),
prodLoc = currentLocation.substring(0,48), 
techLoc = currentLocation.substring(0,47); 

The error: "currentLocation.substring is not a function"

But this part of the code is fine:

var URL = $(this).attr("href").substring(2) + ' #main';

All of the code:

jQuery(function($){

    var siteURL = "http://" + top.location.host.toString() + "/wordpress", // Declareren van URL van de website.
        URL = '', // Declareren van een URL, welke dan ook. 
        currentLocation = '',
        muzLoc = '',
        prodLoc = '',
        techLoc = '',               
        allLinks = $('a[href^=' + siteURL + ']' ), // Declareren van alle menu-links. Het teken ^ betekent 'begint met'.
        otherLinks = $('a[href^=' + siteURL + "/wp-content" + ']'), 
        siteLinks = $(allLinks).not(otherLinks),      
        mainDiv = $("#content"),
        hash = window.location.hash,
        muziekURL = "http://www.elizabet.nl/wordpress/#/muziek_pf/",
        productieURL = "http://www.elizabet.nl/wordpress/#/productie_pf/",
        techniekURL = "http://www.elizabet.nl/wordpress/#/techniek_pf/";    

    if (hash) {
        hash = "/wordpress" + hash.substring(1); // substring methode haalt karakters van je string af. In dit geval de #, vanwege de offset=1.
        URL = hash;
        $(mainDiv).load(URL);           
    }

function pageLoad() {

                var allLinks = $('a[href^=' + siteURL + ']' ),
                otherLinks = $('a[href^=' + siteURL + "/wp-content" + ']'), 
                sit